Two techniques. The first one is pretty straightforward. What I show is actually
a variant where you grab the dagger to apply the lock.

The second technique is a little unclear, because if the attack is from above
as the text suggests, the arms can't end up in the position depicted. Push the
attack down, grab around his neck, throw him backwards.

One way I think the position depicted can be reached, is when the attack is
to the fourth opening. This then looks very similar to one of Marozzo's dagger
defences. (Note relative arm positions)